Due to intensified economic and environmental consequences of burning fossil fuels, increasing amounts of research have been directed towards using radiant solar energy to catalyze or power solar fuels reactions (Steinfeld, R. Palumbo, “Solar Thermochemical Process Technology” in Encyclopedia of Physical Science and Technology; R. A. Meyers Ed., Academic Press, BVol. 15, pp. 237-256, 2001; Kim et. al. Energy Environ. Sci., 5, 8417, 2012; Kim et. al. Energy Environ. Sci., 4, 3122, 2011; Harriman, Phil. Trans. R. Soc. A 371, 20110415, 2013). The environmental problems associated with burning fossil fuels stem from the emission of greenhouse gases. For every ton of burned carbon 3.67 tons of CO2 are released into the atmosphere and CO2 emissions from burning fossil fuels continue to rise and reached almost 40 Gt in 2013 (Foley, “Global Carbon Emissions Projected to Reach Record High in 2013” Nov. 19, 2013 in Nature World News). Meanwhile the radiant solar energy impinging on the earth's surface over one hour is greater than the world's annual energy usage and an alternative solution to the impending energy and climate crises is to make solar fuels from the suns energy rather than continue to deplete legacy fossil fuels from the earth (Izumi, Coord. Chem. Rev. 257, 171, 2013, Neatu et. al., Int. J. Mol. Sci. 15, 5246, 2014, Habisreutinger et. al. Angew. Chem. Int. Ed. 52, 7372, 2013, Navalon et. al., ChemSusChem. 6, 562, 2013).
The concept of solar fuels is based on harnessing an abundant supply of energy from the sun and storing it in the form of chemical bonds as energy rich transportable fuels and chemical feed stocks. The most common solar fuel investigated in the literature is hydrogen gas generated from solar powered water splitting. Other solar fuel reactions involving the reduction of CO2 to generate carbon-based fuels and chemicals, such as carbon monoxide (CO), methane (CH4), and methanol (CH3OH) offer another source of energy with neutral CO2 emissions. Other reactions that reduce CO2 to useful fuels in a hydrogen environment under solar irradiation may be considered as a complementary solar fuels reaction. For example, the Sabatier reaction reduces CO2 to CH4 in a hydrogen environment. This reaction is not a direct solar fuels reaction because it does not increase the amount of energy stored in chemical bonds whether or not it is activated under solar irradiation. However, when coupled with a solar powered water-splitting reaction it can simultaneously reduce greenhouse gas emissions and provide methane to natural gas pipeline networks. Furthermore, CO2 reduction reactions in the gas-phase, rather than in the liquid phase, are expected to provide the most practical and economically feasible route to large-scale solar fuels operations (Olah et. al. J. Am. Chem. Soc., 133, 12881, 2011). In fact, over the last decade increasing amounts of natural gas have been produced through advances in directional drilling and hydraulic fracturing and natural gas power plants have led to reduced emissions of CO2, NOx and SO2. Thus, as shown in FIG. 9, the solar powered photomethanation of CO2 using a renewable source of H2 is a present-day solution that can simultaneously reduce greenhouse gas emissions and also provide methane to natural gas pipeline networks (Lattes, Chemistry International, 35, 5, p. 7-10, ISSN (Online) 1365-2192, ISSN (Print) 0193-6484, DOI: 10.1515/ci-2013-0504, May 2014; de Gouw et. al., Earth's Future, 2: 75, 2014).
Gas phase photomethanation of CO2 with H2 was initially reported using a catalyst comprised of dispersed Ru—RuOx on TiO2 (Thampi et. al., Nature. 327, 506, 1987). Enhanced methanation rates were originally attributed to the chemical effects of electron-hole pairs generated from UV-light absorption in the TiO2 support. However, subsequent studies revealed that photoactive species adsorbed on the catalyst surface (Revilliod et. al., Sol. Energ. Mater. 24, 522, 1991) as well as the increased temperature of the catalyst under light irradiation (Melsheimer et. al., Catal Lett. 11, 157, 1991) played a more significant role in increasing the methanation rates rather than the direct band-gap absorption of the TiO2 support. Since this initial study, numerous catalysts have been tested for photoactivated CO2 reduction with H2. For example, Yoshida et. al. tested TiO2, ZrO2, V2O5, Nb2O5, Ta2O5, WO3, and ZnO and found that of these materials, only ZrO2 exhibited photoactivity for the reduction of CO2 to CO in a H2 atmosphere (Yoshida et. al., Catal Surv Jpn, 4, 2, 2000). In a following study, the photoreduction of CO2 to CO using H2 gas was also observed on the surface of a MgO catalyst (Teramura et. al. J. Phys. Chem. B., 108, 346-354, 2004). The reaction mechanisms for both the MgO and ZrO2 catalysts involved the photoexcitation of carbonaceous species adsorbed on the catalyst surface. Furthermore, Lo et. al. also demonstrated the photoreduction of CO2 over ZrO2 in a circulating photocatalytic reactor (Lo et. al., Sol. Energ. Mat. Sol. C., 91, 1765, 2007). More recently, CO2 photoreduction to methanol has been reported to occur over Graphene Oxide (GO) catalysts (Hsu et. al. and, L. C. Chen, Y. C. Lin, K. H. Chen, Nanoscale, 5, 262, 2013). The absorption edge of the GO catalyst was at least 3.2 eV and it was proposed that the reaction mechanism involves photogenerated electrons and holes migrating to the GO surface and reacting with adsorbed CO2 and H2O to produce methanol. CO2 photoreduction to methanol was also reported over zinc-copper-gallium layered double hydroxides (K. Teramura et. al. Chem. Phys. Lett. 467, 191, 2008) and it was suggested that CO2 reacted with hydroxyl groups bound to Cu to form hydrogen carbonate which subsequently decomposed in an H2 atmosphere under UV-Visible light. Moreover, very recently Hoch et. al. have shown that hydroxylated indium oxide nanoparticles with a bixbyite crystal structure and forbidden electronic band gap are active for the photoreduction of CO2 to CO. The proposed reaction mechanism involves oxygen vacancies and hydroxides at the surface of the nanoparticles to reduce CO2 (Hoch et. al., submitted for publication 2014).
In general, when testing catalysts for the photoactive reduction of CO2 it is important to ensure that the products do not originate from adventitious carbon sources (C. Yang, J. Am. Chem. Soc., 132, 8398, 2010). In this context, isotope tracing experiments using Fourier-Transform Infra-Red (FTIR) spectroscopy and Mass Spectroscopy (MS) are particularly effective (Y. Izumi, Coordin. Chem. Rev. 257(1), 171-186, 2013). Further, it is interesting to note that CO2 photoreduction rates reported in the literature for catalysts tested using isotope tracing experiments are on the order of 1 μmol/gcat·h or less, orders of magnitude below that required for the technological development of a practical large scale CO2 photoreduction process. However, very recently these poor performance metrics were broken when Sastre et. al. reported the complete photocatalytic reduction of CO2 to methane in H2 using a catalyst comprised of Ni on a silica-alumina support (Sastre et. al., J. Am. Chem. Soc. 136, 6798-6801, 2014). The complete methanation of CO2 reported in this work infers a CO2 photoreduction rate well over 10 mmol/gcat·h. It was proposed that the reaction mechanism involves photogenerated electrons (holes), reducing (oxidizing) H2 to form Ni—H which then functions as the active CO2 reducing agent. Moreover, by performing experiments with optical filters it was determined that 76% of the photoreduction of CO2 was activated using UV light, which is consistent with the photon energy required to excite electrons across the 3.8 eV bandgap of NiO (R. J. Powell et. al., Phys. Rev. B2, 2182, 1970). It is also noteworthy that this proposed mechanism is reinforced by previous experiments reporting the methanation of CO2 over NiO-based catalysts that were pre-treated in an H2 atmosphere under UV-light. (K. Ogura, et. al., J Mol Catal. 72, 173-179, 1992). In this regard it is noteworthy that the photon energy required to excite electrons across the ˜3.8 eV bandgap of NiO is about 330 nm. In another set of experiments recently reported in the literature it is shown that the Sabatier reaction on Ru-based catalysts with Al2O3 supports proceeds photothermally. Furthermore, the results from this study show that the Ru-based catalyst with an Al2O3 support does not exhibit any photochemical activity (Meng et. al., Angew. Chem. 2014, 126, 1-6).
Note that in all the aforementioned research the support was absorbing in the ultraviolet wavelength region of the solar spectrum but transparent to the rest of the solar spectrum in the visible and infrared range and therefore distinct to a solar selective catalyst support which is the central focus of the invention described herein.
All references listed herein are incorporated by reference herein in their entireties.